The present invention relates to a mechanical press, and in particular to a guiding arrangement for the reciprocating slide of such a press.
A typical press of the mechanical variety comprises a bed which is mounted to a platform or the floor of the shop, a vertically spaced crown portion in which the drive assembly for the slide is contained, and one or more uprights rigidly connecting the bed and crown and maintaining the bed and crown in vertically spaced relationship. The crown contains the drive assembly, which typically comprises a crankshaft having a plurality of eccentrics thereon and connections connected to the eccentrics of the crankshaft at their upper ends and to the slide at their lower ends. The slide is mounted within the uprights for vertical reciprocating motion and is adapted to have the upper half of the die set mounted to it with the other half mounted to the bolster, which is connected to the bed.
At one end of the crankshaft there is usually mounted a flywheel and clutch assembly wherein the flywheel is connected by a belt to the output pulley of the motor so that when the motor is energized, the massive flywheel rotates continuously. When the clutch is energized, the rotary motion of the flywheel is transmitted to the crankshaft which causes the connections to undergo rotary-oscillatory motion that is transmitted to the slide assembly by means of a wrist pin, for example, so that the rotary-oscillatory motion is converted to straight reciprocating motion. The connections may be connected directly to the slide or connected by means of pistons which are in turn slidably received within cylinders connected to the crown. An example of this latter type of construction is disclosed in U.S. Pat. No. 3,858,432, which is owned by the assignee of the present application.
The slide is usually mounted in the space defined by the crown, bed and uprights and is very accurately guided for rectilinear movement in the vertical direction in the case of a straight sided press, and in a direction slightly inclined rearwardly from vertical in the case of an open back inclined press. The slide must move very accurately along its axis of reciprocation in order to ensure that the two die halves mate properly during stamping or forming of the part. One very commonly used guiding system comprises a plurality of gibs mounted directly to the uprights, the gibs having very accurately machined and oriented guide surfaces that engage the slide as it reciprocates vertically. In this type of guiding system, as in most other guiding systems, the die sets also include guide pins which provide additional guiding of one die half relative to the other in addition to the guiding of the slide which is built into the press itself.
A further type of slide guiding comprises a plurality of guideposts rigidly connected to the crown and bed and extending in the vertical direction. The slide includes bushings or the like that slide over the guideposts and are typically machined with very close tolerances so that there is adequate stiffness to cause the slide to reciprocate in the proper direction. A problem with this type of guiding system, however, is that the guideposts are quite long since they must extend from their points of attachment on the crown and bed, and this length makes it quite difficult to achieve and maintain the parallelism which is necessary between them so that the slide can reciprocate accurately and without binding. Such guideposts are also expensive to manufacture because of their long length and the fact that they must be an accurately machined part. A further prior art type of slide guiding system comprises a plurality of guide bushings rigidly mounted to the bed of the press but terminating short of the crown so that they are supported in a cantilevered fashion. Guideposts are then rigidly connected to the slide and extend downwardly and into the guide bushings. Because the guide bushings are mounted only at one end to the bed yet must extend upwardly a considerable distance in order to meet the guideposts connected to the crown, results in a large cantilevered load at the end of the bushings, and it is quite difficult to maintain the accuracy and stiffness which is necessary at these distances from the point of mounting the bushings to the bed. A still further disadvantage to the cantilevered bushing arrangement is the cost involved, since the bushings must be quite massive and accurately machined along their length.
A problem which has been experienced in the past in connection with mechanical presses is the tendency of the slide to be tilted in the front to back direction about a horizontal axis. Although the slide itself is guided, either by the gib, guidepost or guide bushing arrangements described above, as the crankshaft eccentric rotates beyond its top dead center position, the force exerted by the connections on the slide is no longer completely vertical, but includes a horizontal component of force which increases as the eccentric moves to the 90.degree. past top dead center position. This horizontal component of force tends to cause the slide to tilt about a horizontal axis thereby resulting in nonparallelism between the slide and bolster which can cause improper stamping or forming of the part. Additionally, increased wear on the guide bearing surfaces is likely to occur. In order to overcome this tilting motion, prior art presses having employed bearings that engage the ram or connection itself. A disadvantage to this system wherein the ram or connection is guided by means of bearings in the crown but the slide itself is guided by means of guideposts or bearings which are connected, at least at one end, to the bed portion of the press frame is that there are two spatially separated frames of reference for the guide system, one on the crown and one on the bed. In the aforementioned U.S. Pat. No. 3,858,432, the pistons that connect the connections to the slide are centered within their respective cylinders by means of a plurality of hydrostatic bearings located around the periphery. Although this provides a very good bearing for the connections, the slide itself is not directly guided, and the only other structure contributing to the guiding may be the guide pins associated with the die halves.